Installation for very long term storage of heat-generating products such as nuclear waste

ABSTRACT

A very long term storage installation for calorific products such as nuclear waste, comprises at least one closed cavity ( 10 ), in which at least one product confinement container is housed ( 14 ). To evacuate the heat released by the stored products, each container ( 14 ) is surrounded by a jacket ( 26 ) associated with a thermosiphon ( 24 ) whose cold source is formed of an air condenser provided above a slab ( 20 ) sealing the top part of the cavity. The jacket ( 26 ) is preferably interchangeable and tightly surrounds the container ( 14 ).

FIELD OF THE INVENTION

The invention concerns an installation intended to ensure the storageover the very long term of calorific products likely to release largequantities of heat which may decrease in time.

The term “storage” designates the reversible storage of packagedproducts, accompanied by evacuation of the heat released by theseproducts. By the adjective “reversible” is meant that the storedproducts may be taken out from storage.

The expression “very long-term” means at least fifty years and,preferably, several periods of fifty years.

One privileged application of the installation of the invention concernsthe storage of nuclear waste having very high long-term activity, suchas irradiated fuel in nuclear reactors.

PRIOR ART

The storage of hazardous calorific products such as nuclear waste is amajor problem for which a certain number of solutions have already beenput forward.

Among such solutions, reference will only be made to those which ensurepassive cooling of the products without the supply of any outsideenergy. This passive form appears essential to obtain the requiredreliability throughout the very long period of storage underconsideration.

According to one first known storage technique, the products arepackaged in containers which are placed in cavities made in the ground,said cavities being delimited by concrete walls. An air-filled space isprovided between each container and the cavity walls. Heat evacuation isobtained solely by circulation of air under natural convection.

One notable disadvantage of said type of installation is that cooling isachieved via a primary circuit, in direct contact with the containerwalls. This type of arrangement is dispersive in the event of anincident and therefore dangerous for the environment. In addition, itonly allows very limited evacuation of the heat flow.

According to another known storage technique, the general arrangement issimilar to the previous one, but cooling is ensured by secondary coolingcircuits through which a fluid passes, water in particular or air undernatural convection. These circuits are fully embedded in the concretewalls which delimit the cavities housing the containers.

Such installations have a certain number of disadvantages.

Firstly, since cooling is achieved only inside the concrete wallsthemselves, the surfaces of these walls delimiting the cavities areheated directly by the stored products. The consequence is weakening ofthe concrete at least on the surface. Also, the temperature of thecontainers remains very high, leading to rapid ageing of their welds.Finally, with such storage installations it is not possible to controlthe outside temperature and therefore the inside temperature of thecontainers and this may, for example, lead to destruction of thecladding of the irradiated fuel.

A third known storage technique sets itself apart from the previoustechnique chiefly through the fact that the secondary cooling circuitscross through the walls delimiting the cavities and are partly locatedin the space surrounding the containers.

In this case, almost the same disadvantages are found as with theprevious known technique. Also, since the cooling circuit passes locallythrough the surfaces of the concrete walls delimiting the cavities,these surfaces are subjected to non-homogeneous heat stresses which leadto accelerated ageing of the concrete.

With a fourth known storage technique, the space provided between eachcontainer and the cavity in which it is housed is filled with water andthe cooling circuit is fully located in this space.

This known solution is characterized by corrosion problems due to thefact that the containers are immersed in water. Also, any leak from thecooling circuit entails a contamination risk if the stored products arenuclear waste. Further, the maintenance of this type of storage deviceis particularly heavy.

From document DD-A-223 562 an installation is known for the storage ofirradiated nuclear fuel in which cylindrical containers containing theproducts are placed one on top of the other in wells delimited byconcrete walls. The wall of each well is lined on the inside with ametal tube which projects above the well as far as a heat dissipater,with vanes or similar, able to transmit the heat it receives to thesurrounding atmosphere. A plug is placed at the top of the well insidethe metal tube above the containers.

The efficacy of said device is relatively limited and does not preventmajor heating of the containers and well walls. Also, a substantial heatgradient exists between the containers placed at the bottom of the welland the containers nearer to the surface. Consequently, surfaceweakening of the concrete and accelerated ageing of the container weldsand dissipater tube (which is not interchangeable) are practicallyunavoidable.

Also, document U.S. Pat. No. 4,040,480 describes a storage installationfor radioactive products in which the products are packaged incylindrical containers and placed in a ring-shaped cavity delimitedbetween the concrete wall of a well having a circular cross-section anda closed vertical tube, forming a coolant duct, positioned in the wellaxis. At its top part, positioned above a plug sealing the well, thevertical tube carries cooling vanes in contact with the air.

The heat diffused by the stored products propagates both towards thewell wall and towards the tube forming the coolant duct. Relativelyrapid damage to the concrete surface is therefore predictable. Also, noprovision is made in the event of failure of the coolant duct.

As a general rule, the installations known to date are designed for amaximum lifetime of approximately fifty years, whereas the need existsin the nuclear industry for storage over several fifty-year periods,typically up to 300 years.

Document JP-A-05 273393 suggests packaging spent fuel assembliesseparately in casings and placing each of the casings in a closedcontainer hung from a slab of a building. The lower part of eachcontainer is housed in an individual well and the top part is positionedin a common corridor swept by a stream of coolant air.

Finally, document FR-A-2 160 concerns a transport tower for radioactiveproducts surrounded by a jacket fitted with cooling vanes, the jacketbeing assembled such that it can be dismounted.

DISCLOSURE OF THE INVENTION

The subject of the present invention is precisely a storage installationfor calorific products, such as nuclear waste, which does not have thedisadvantages of installations of the prior art. In other words, thesubject of the invention is a passive storage installation able toevacuate a great quantity of heat over a very long period, whileoffering very high reliability and sturdiness, in particular by onlysubjecting the materials to demands that are compatible with a very longlifetime.

In accordance with the invention, this result is obtained by means of avery long-term storage installation for calorific products, comprisingat least one sealed cavity, at least one confinement container for saidproducts, able to be housed in the cavity, and means forming athermosiphon able to dissipate the heat released by said products abovethe cavity, characterized in that the thermosiphon-forming means arepartly integrated in a jacket in direct contact with the container whichit surrounds.

The use of means forming a thermosiphon integrated into a jacket closelysurrounding the container makes it possible to provide efficientevacuation of the heat released by the products contained in thecontainer, without however risking any dispersion of contamination inthe event of an accident. Also, the jacket forms a heat shield betweenthe container and the wall of the cavity. The latter, generally made inconcrete if the stored products are nuclear waste, is therefore cooledefficiently and in homogeneous manner in the same way as the actualcontainer. Accelerated ageing of the concrete, container welds andcontainer contents is therefore avoided. In addition, it is possible tohave knowledge of and efficiently adjust the surface temperature of thecontainer and the temperature of the wall of the well or trench. Thisalso makes it possible to pilot the conditions of storage in accordancewith usual hypotheses (not generally heeded in existing installations)according to which the temperature of the concrete surface is known andfixed. Such installation also has the advantage of allowing the coldsource, positioned above the cavity, to be adapted to changes over timein the heat released by the stored products.

In one preferred embodiment of the invention, the jacket can bedismounted. Also, the cavity is advantageously sealed by a removableplug above the container. With this arrangement it is possible, ifnecessary, to replace the jacket integrating the thermosiphon or toremove the container should any problems arise.

In this case, the jacket is advantageously open and made in a flexible,elastic material such as metal so that it can occupy a natural state inwhich it is spaced away from the container. In this natural state thejacket can be easily mounted and dismounted. In this case, releasableclamp means are provided, to apply the jacket tightly around thecontainer at the time of placing in storage.

Preferably, the jacket is then in the shape of a cylinder open along agenerating line and the releasable clamp means are positioned betweenthe edges vis-a-vis this generating line.

In order to avoid excessive heating of the cavity walls, a spacegenerally filled with air is advantageously provided inside the cavityaround the container fitted with its jacket, and the air may or may notbe circulated by natural convection.

In the preferred embodiment of the invention, the jacket comprises aplurality of outer tubes filled with coolant fluid whose lower and upperends respectively lead to a lower ring collector and an upper ringcollector.

In this case, cooling vanes are preferably formed on at least some ofthe outer tubes, such as to increase heat exchange with the aircontained in the cavity.

In this preferred embodiment of the invention, the outer tubes may bewelded to the jacket.

As a variant, the jacket may also comprise a plurality of segments,fixed end to end by assembly means such as welds or rivets. Each of theouter tubes is then made in one piece with one of these segments.

In order to ensure cooling of the fluid (generally water) contained inthe thermosiphon-forming means, the latter also comprise heat exchangemeans placed above the cavity and forming a cold source.

If the jacket can be dismounted, the heat exchange means are connectedto the jacket by connection means which can be disconnected.

Advantageously, the heat exchange means are adapted to variations in theflow of heat to be dissipated.

In the preferred embodiment of the invention, the thermosiphon-formingmeans form a coolant duct.

Advantageously, the installation of the invention is applied to thestorage of nuclear waste. In this case, the cavity is delimited byconcrete walls.

BRIEF DESCRIPTION OF THE FIGURES

Using non-restrictive examples the different embodiments of theinvention are described below with reference to the appended drawings,in which:

FIG. 1 is a vertical section view giving a very schematic diagram of astorage installation for calorific products according to the invention;

FIG. 2 is a perspective, partial cut-away section view showing the upperpart of the jacket which tightly surrounds each container in theinstallation of FIG. 1.

FIG. 3 is a section view along a horizontal plane showing the jacket asa solid line in its natural open state and as a dashed line when ittightly surrounds the container;

FIG. 4 is a section view along a horizontal plane illustrating anotherembodiment of the jacket on a larger scale; and

FIGS. 5 and 6 are section views comparable with FIG. 4, illustratingvariants of embodiment of the invention.

DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS OF THE INVENTION

FIG. 1 is a very schematic diagram of part of a very long-term storageinstallation for calorific products, such as nuclear waste, built inaccordance with the invention.

The installation comprises at least one sealed cavity 10, such as anembedded trench, whose sides and bottom are delimited by concrete walls12.

In the described embodiment, cavity 10 is in the form of a rectilinearembedded trench. This trench is able to house several containers 14 inwhich the products to be stored are packaged. However, the shape ofcavity 10 may be different while remaining within the scope of theinvention. Therefore, it is possible for each of containers 14 to beplaced in a separate, individual cavity.

In similar manner, the containers 14 used to confine the products to bestored are metal containers whose size and shape may vary whileremaining within the scope of the invention. In the embodimentillustrated by way of example, the containers 14 are of cylindricalshape and are placed side by side and on one same level in the trenchforming cavity 10, their axes being substantially oriented in verticaldirection.

More precisely, each of containers 14 is in contact neither withneighbouring containers nor with the walls of cavity 10. In other words,a space 16 filled with air is provided, inside cavity 10, around each ofcontainers 14. Air circulation in this space 16, by natural convection,contributes towards the cooling of containers 14.

To preserve this space below each of containers 14, the latter rest onthe bottom of cavity 10 via a pedestal 18. Also positioning or spacingmeans (not shown) are advantageously provided between cavity 10 and eachof containers 14, in order to ensure the positioning and centering ofthe containers in the cavity.

As illustrated in FIG. 1 also, cavity 10 is sealed at the top by aconcrete slab 20. Above each of containers 14, the concrete slab 20 hasan opening generally of round shape sealed by a removable plug 22. Thisremovable plug 22 is also made in concrete. Its removal makes itpossible to place each of containers 14 in position separately insidecavity 10, and optionally to remove them from this cavity. For thispurpose, handling means (not shown) are provided above the concrete slab20. This arrangement ensures biological protection when the storedproducts are nuclear waste products, and mechanical protection againstfalling aircraft or acts of malevolence.

To evacuate the heat released by the products stored inside a container(which may represent an energy of 80 kW) into atmospheric air, theinstallation of the invention also comprises means 24 forming athermosiphon (FIG. 2). More precisely, part of thesethermosiphon-forming means are integrated in a jacket 26 which surroundseach of containers 14 such that its smooth inner cylindrical surface 27is normally in close contact with the smooth outer cylindrical surface15 of the container. In addition, the jacket 26 is made in a heatconductive material, a metal for example such as stainless steel orcopper.

With this arrangement, the heat released by the products contained incontainers 14 is transmitted by the thermosiphon-forming means 24 inefficient and homogeneous manner over the entire periphery of thesecontainers. Heat contact between the container and the jacket is ensuredby the direct contact between the two walls. Heat resistance is reducedsince the effective thickness of the film of residual air between thewalls is limited to a fraction of a millimetre.

In the embodiment illustrated in the figures, the part of thethermosiphon-forming means integrated into jacket 26 is in the form of asealed cooling circuit surrounding container 14. This circuit comprisesa plurality of outer tubes 28, fixed to the outer surface of the jacket26 along its generating lines, and a lower ring collector 30 and anupper ring collector 32, to which the lower and upper ends of the tubes28 respectively lead. Tubes 28 are numerous and evenly distributedaround the entire circumference of jacket 26. A coolant fluid, such aswater at 100 C, is placed inside the circuit. When in operation, thewater is in the liquid state in the lower ring collector 30 and in thevapour state in the upper ring collector 32. The thermosiphon-formingmeans 24 therefore form a coolant duct which tightly surrounds thecontainer and homogenizes the temperature thereby preventing theformation of hot points.

In other words, the thermosiphon-forming means 24 use the principle ofthe evaporation/condensation cycle of a coolant fluid to transfer heatfrom a hot source, formed by container 14, towards a cold source placedabove slab 20. They are sealed, passive means since they only act bychange in fluid phase.

As schematically shown in FIG. 1, the cold source of thethermosiphon-forming means 24 comprises heat exchange means 34 such asan air condenser positioned outside and above cavity 10, that is to sayabove the concrete slab 20. These heat exchange means 34 are connectedby two pipes 36 to collectors 30 and 32 of the cooling circuitassociated with jacket 26. More precisely, in the embodiment describedby way of example, one same heat exchange means 34 is connected to eachof the cooling circuits carried by jackets 26 surrounding all thecontainers 14 placed in one same cavity 10.

The heat exchange means 34 may be of any shape appropriate for theirfunction and still remain within the scope of the invention. It is to benoted that they may be implanted at a certain height above the concreteslab. 20 and at a certain distance from the containers with no notabledeterioration in the performance of the installation.

The pipes 36 which connect the heat exchange means 34 to the lower 30and upper 32 ring collectors of one or more cooling circuits associatedwith jackets 26 cross through passageways provided for this purpose inthe removable plugs 22.

In the preferred embodiments of the invention illustrated in thefigures, the jackets 26 are mounted on the containers such that they maybe dismounted separately from the latter. It is therefore possible,after removing any one of the removable plugs 22, to replace the jacket26 of the corresponding container 14 without it being necessary toremove the container from cavity 10. The sizes of the opening made inthe slab 20 above each of containers 14 is adapted to allow suchreplacement.

This arrangement greatly facilitates the very long term management ofthe storage installation. It allows easy servicing of any faulty part ofthis installation using remote handling means placed over slab 20,guaranteeing the very long-term evacuation of the heat dissipated by theproducts stored in the containers.

In practice, and as illustrated in particular by FIGS. 2 and 3, thepossible dismounting of jackets 26 is achieved by making each one in theform of a cylinder open along a generating line. In addition, thejackets 26 are made of flexible, elastic material having very lowoverall stiffness such as a metal sheet of narrow thickness (3 to 4 mmfor example).

In its natural state at rest, and as shown by the solid line in FIG. 3,the diameter of the smooth inner cylinder surface 27 of jacket 26 ismuch greater than the diameter of the smooth outer cylinder surface 15of container 14. Therefore, there is a gap between jacket 26 andcontainer 14 when the jacket is in its natural state at rest. It canconsequently be easily dismounted or positioned around a container 14placed in cavity 10 through a movement made parallel to the verticalaxis of the container.

As shown in FIG. 2 in particular, each of the edges opposite the opengenerating line of jacket 26 comprises a clamp plate 38 radiallyoriented outwards so that the two plates 38 are substantially parallelto one another. The plates 38 of one same jacket 26 have holes atregular intervals in which bolts 40 can be mounted forming releasableclamp means, able to apply jacket 26 tight against container 14.

The bolts 40 which here form the releasable clamp means may be replacedby any other means able to bring together plates 38 in order to applythe smooth inner cylindrical surface 27 of jacket 26 against the smoothouter cylindrical surface 15 of container 14 by tautening the jacket.This result may be obtained without any excessive effort on account ofthe weak stiffness of the material in which jacket 26 is made.

It is to be noted that the releasable clamp means are preferably chosenso that they can be easily placed in position and actuated by remotehandling means from the space located above slab 20 after removing plug22 or a shutter provided in the latter.

The heat exchange means 34 are advantageously arranged so that they canbe adapted to changes over time in the flow of heat released by theproducts stored in the containers. However, any servicing of jackets 26must be feasible even though said heat exchange means 34 are inposition. Therefore, the positioning of these heat exchange means 34above the concrete slab 20 must be made so that replacement of jackets26 is possible and so that containers 14 can be put in place andoptionally removed.

As also illustrated in FIG. 1, the arrangement just described leads tomaking provision for connection means 42, which can be disconnected, oneach of pipes 36. These disconnectable connection means 42 areadvantageously positioned under slab 20. They are accessible, as are thereleasable clamp means, via access points provided in the removableplugs 22. Within the scope of the invention, the disconnectableconnection means 42 may be in any form.

According to a first embodiment of the invention, illustrated in FIGS. 2and 3, the jacket 26 is made in a relatively thin, flexible metal sheet,and tubes 28 are directly welded to the outer surface of this sheet.

Under another embodiment of the invention, schematically shown in FIG.4, jacket 26 is formed of a plurality of segments 26 a, placedcircumferentially end to end. Each of segments 26 a is fixed to theadjacent segment by assembly means formed in this case by welds 44.

In this embodiment shown in FIG. 4, each of the outer tubes 28 is madein one single piece with a corresponding segment 26 a of jacket 26.

FIG. 5 illustrates a variant of the embodiment in FIG. 4, which differsessentially in the assembly means joining the different segments 26 aend to end forming jacket 26. In this case, instead of being joined bywelds 44, segments 26 a have superimposed adjacent edges through whichthe fixing parts are passed such as rivets as shown by the dashed lines44′ in FIG. 5.

FIG. 6 illustrates another variant of the jacket 26. It is to be notedthat this variant can be applied indifferently to the embodiments whichhave just been described with reference to FIGS. 2, 4 and 5successively, even though FIG. 6 only illustrates the case in FIG. 5.

As illustrated in FIG. 6, each of outer tubes 28 is, in this case,provided with at least one cooling vane 46. This vane 46, placed in thespace 16 arranged in cavity 10 around jacket 26, improves the “vaneeffect” provided by the actual tubes 28. This “vane effect” enablesevacuation of the heat released by the products stored in thecontainers, in combination with natural air circulation in space 16surrounding the containers, when this type of cooling becomessufficient, in the event of a decrease over time in the flow of heatfrom the stored products. In addition, this “vane effect” facilitatesthe emergency cooling of the container in the event of failure of thethermosiphon-forming means.

The storage installation just described provides for the storage,confinement and evacuation of the heat dissipated by calorific productsover a very long period. The thermosiphon-forming means 24 allow a largequantity of heat to be evacuated, as is required at the start of thestorage period of nuclear waste. The proposed arrangement thereforemakes it possible to maintain the welds of container 14 and thecalorific products at a temperature that is sufficiently low to preventtheir accelerated ageing. It also makes possible the application of ahomogeneous temperature to the surface of the concrete cavity which isalso sufficiently low to prevent its weakening over time.

In addition, the thermosiphon-forming means 24 form a secondary circuit,separated from the products packaged in the container both by its walland by the walls of tubes 28 carried by jacket 26. This ensuresenvironmental protection in the event of a container leak.

Also, in the preferred embodiments of the invention according to whichjacket 26 can be dismounted, it is possible to act quickly and withoutdanger on the thermosiphon-forming means by direct replacement of thefaulty jacket.

It is to be noted that the installation may be completed by additionalequipment (not shown) with which to collect any possible liquid or gaseffluent and to ensure its control before it is discarded so as toprotect the environment. Such equipment is conventional and does notcall for any particular description.

Evidently the invention is not restricted to the embodiments justdescribed by way of example, but covers all their variants. Hence, ifthe storage period is not too long, the jackets can be clamped and fixedpermanently to the containers. On the contrary, the interchangeabilityof the jackets may be achieved by making them in the form of semi-shellsassembled together in dismountable manner, or semi-shells articulated toone another, or in any other appropriate form providing close contactbetween the jackets and the containers, able to ensure optimal heatexchange.

In addition, the cooling circuit associated with the jacket may be madedifferently, for example in the form of spiral-shaped tubes orpassage-ways incorporated in thicker areas of the jacket.

What is claimed is:
 1. A very long-term storage installation forcalorific products, comprising at least one sealed cavity, at least onecontainer to confine said products, said container being housed in saidcavity, and thermosiphon-forming means able to dissipate above thecavity the heat released by said products, wherein a jacket surroundsand is in direct contact with the container, said jacket partlyintegrating the thermosiphon-forming means, wherein a space is providedinside the cavity, around the container fitted with the jacket.
 2. Theinstallation according to claim 1, wherein said jacket can be removedfrom the container.
 3. The installation according to claim 2, whereinsaid jacket is open and made in a flexible, elastic material andoccupies a natural state spaced away from the container, whereinreleasable clamp means being provided to apply the jacket tightlyagainst the container.
 4. The installation according to claim 3, whereinsaid jacket is in a cylinder form and open along a generating line, andthe releasable clamp means are inserted between edges opposite thegenerating line.
 5. The installation according to claim 1, wherein thecavity is sealed by a removable plug above the container.
 6. Theinstallation according to claim 1, wherein the jacket comprises aplurality of outer tubes filled with coolant liquid and leadingrespectively to a lower ring collector and an upper ring collector. 7.The installation according to claim 6, wherein the outer tubes arewelded to the jacket.
 8. The installation according to claim 2, whereinthe thermosiphon-forming means also comprise heat exchange means placedabove the cavity.
 9. The installation according to claim 8, wherein theheat exchange means are connected to the jacket by disconnectableconnection means.
 10. The installation according to claim 8, wherein theheat exchange means are adapted to variations in the flow of heat to bedissipated.
 11. The installation according to claim 1, wherein thethermosiphon-forming means form a coolant duct.
 12. The installationaccording to claim 1, applied to storage of nuclear waste, wherein thecavity is delimited by concrete walls.
 13. A very long-term storageinstallation for calorific products, comprising at least one sealedcavity, at least one container to confine said products, said containerbeing housed in said cavity, and thermosiphon-forming means able todissipate above the cavity the heat released by said products, wherein ajacket surrounds and is in direct contact with the container, saidjacket partly integrating the thermosiphon-forming means, wherein thejacket comprises a plurality of outer tubes filled with coolant liquidand leading respectively to a lower ring collector and an upper ringcollector, wherein the cooling vanes are formed on at least some of theouter tubes.
 14. A very long-term storage installation for calorificproducts, comprising at least one sealed cavity, at least one containerto confine said products, said container being housed in said cavity,and thermosiphon-forming means able to dissipate above the cavity theheat released by said products, wherein a jacket surrounds and is indirect contact with the container, said jacket partly integrating thethermosiphon-forming means, wherein the jacket comprises a plurality ofouter tubes filled with coolant liquid and leading respectively to alower ring collector and an upper ring collector, wherein the jacketcomprises a plurality of segments fixed end to end by assembly means,each of the outer tubes being made in a single piece with one of saidsegments.